1. Field of the Invention
The present invention relates to a working vehicle configured so that rotational power whose speed has been changed in a non-stepwise manner by an HST is differentially transmitted to a pair of first and second driving wheels on left and right sides through a differential gear device.
Further, the present invention relates to an HST unit including a hydraulic pump main body and a hydraulic motor main body.
2. Background Art
A travel-system transmission structure configured so that rotational power whose speed has been changed in a non-stepwise manner by an HST is differentially transmitted to a pair of main driving wheels through a differential gear device has been proposed and used in a working vehicle such as a tractor (refer to, for example, U.S. Pat. No. 6,993,906).
However, the travel-system transmission structure described in the patent document has a problem of difficulty in providing a free space between the pair of main driving wheels.
Specifically, in the travel-system transmission structure described in the patent document, the differential gear device is accommodated in an axle case for accommodating a pair of left and right driving axles at a substantially-center portion in a vehicle widthwise direction and, further, a casing of the HST is integrated with the axle case at a substantially-center portion in the vehicle widthwise direction.
Accordingly, it is impossible to provide a free space between the pair of main driving wheels and, for example, in a case where a discharge duct is provided between the pair of main driving wheels, it is necessary to form the discharge duct to have a complicated shape, in order to prevent the discharge duct from interfering with the HST and the portion of the axle case which accommodates the differential gear device.
Further, an HST unit capable of changing in a non-stepwise manner a speed of rotational power from a driving power source and outputting the same has been widely used in a travel-system transmission path in a working vehicle such as a tractor.
More specifically, the HST unit includes a pump shaft operatively coupled to a driving power source, a hydraulic pump main body which is supported by the pump shaft in a relatively non-rotatable manner with respect thereto, a hydraulic motor main body which is fluidly connected to the hydraulic pump main body through a pair of HST lines, a motor shaft which supports the hydraulic motor main body in a relatively non-rotatable manner with respect thereto, a capacity adjustment mechanism which changes the capacity of at least one of the hydraulic pump main body and the hydraulic motor main body, and an HST case which accommodates the hydraulic pump main body, the hydraulic motor main body and the capacity adjustment mechanism in a liquid tight manner. Further, the HST unit is structured to be capable of outputting rotational power directly or indirectly from the motor shaft.
By the way, the working vehicle includes a two-wheel hydraulic drive type configured so that a single or a pair of wheels which are placed on a side opposite from a pair of main driving wheels in a vehicle lengthwise direction functions as a non-driving wheel, and a four-wheel hydraulic drive types configured so that the wheels which on the side opposite from the pair of main driving wheels in the vehicle lengthwise direction are driven by a sub hydraulic motor main body separate from the hydraulic motor main body of the HST unit.
However, the conventional HST unit is merely structured to be capable of outputting rotational power directly or indirectly from the motor shaft and, therefore, can be applied to only a working vehicle of the two-wheel hydraulic drive type.
Accordingly, in a case where the conventional HST unit is applied to working vehicle of the four-wheel hydraulic drive type, there is a need for providing an additional hydraulic pump main body for hydraulically driving the sub hydraulic motor main body.